Second Schottky contact metal layer to improve GaN Schottky diode performance

ABSTRACT

A Schottky contact is disposed atop a surface of a semiconductor. A first Schottky contact metal layer is disposed atop a first portion of the semiconductor surface. A second Schottky contact metal is disposed atop a second portion of the surface layer and adjoins the first Schottky contact metal layer. The first Schottky contact metal layer has a lower work function than the second Schottky contact metal layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. Provisional Application No.60/736,893, filed Nov. 15, 2005, the disclosure of which is incorporatedby reference herein.

BACKGROUND OF THE INVENTION

The invention is directed to Schottky diodes as well as to other deviceshaving a Schottky contact and, more particularly, to the Schottkycontact metals used in such devices.

Two important properties of a Schottky diode are its forward voltagedrop, V_(F), and its reverse blocking voltage V_(R). The metal thatforms the Schottky contact in the Schottky diode greatly impacts the twoparameters. For high blocking voltage applications, the Schottky dioderequires a Schottky contact metal having a high work function to providea large barrier height at the metal-to-semiconductor interface. However,the large barrier height also causes a higher voltage drop when thediode is forward biased. Another concern is that when the diode isreverse biased, the highest electric fields occur at the edge of themetal contact, whereas when the diode is forward biased, all of thecontact area conducts the current uniformly.

Known solutions for improving the reverse bias characteristics of GaNSchottky devices also sacrifice the forward bias performance of thedevice, such as by increasing the forward voltage drop. It is thereforedesirable to improve the reverse bias characteristics of such deviceswithout degrading the forward voltage drop.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a Schottky contact is disposedatop a surface of a semiconductor. A first Schottky contact metal isdisposed atop of a first portion of the semiconductor surface. A secondSchottky contact metal layer is disposed atop of a second portion of thesemiconductor surface and at least adjoins the first Schottky contactmetal layer. The first Schottky contact metal layer has a lower workfunction than that of the second Schottky metal contact layer.

In accordance with this aspect of the invention, a Schottky diodeincludes a lower layer of nitride semiconductor disposed atop asubstrate. An upper layer of nitride semiconductor is disposed atop atleast a portion of the lower layer of nitride semiconductor. The lowerlayer of nitride semiconductor is more highly doped than the upper layerof nitride semiconductor. A Schottky contact is disposed atop the upperlayer of semiconductor in the manner described above. The semiconductorsurface being a surface of upper layer of nitride semiconductor. Afurther metal contact layer is disposed atop the lower layer of nitridesemiconductor such that an ohmic contact is formed.

Also according to this aspect of the invention, a field effecttransistor (FET) includes a lower layer of nitride semiconductor isdisposed atop a substrate. An upper layer of nitride semiconductor isdisposed atop at least a portion of the lower layer of nitridesemiconductor. The upper layer is a different nitride semiconductor thanthe lower layer so that a heterojunction is formed between the layers. ASchottky contact is disposed atop the upper layer of nitridesemiconductor in the manner described above. The semiconductor surfaceis a surface of the upper layer of nitride semiconductor. A furthermetal contact layer is disposed atop the lower layer of nitridesemiconductor such that an ohmic contact is formed.

A further aspect of the invention is a method of forming a Schottkycontact atop a surface of a semiconductor. A first Schottky contactmetal layer is formed atop a first portion of the semiconductor surface.A second Schottky contact metal layer is formed atop of a second portionof the surface and at least adjoining the first Schottky contact metallayer. The first Schottky contact metal layer has a lower work functionthan the second Schottky contact metal layer.

A method of forming a Schottky diode and a method of forming a fieldeffect transistor (FET) are also in accordance with this aspect of theinvention.

Another aspect of the invention is a method of improving metal adhesionin a Schottky contact. A first Schottky contact metal layer is formedatop at least a portion of a surface of the semiconductor structure. Thefirst Schottky contact metal layer includes a higher work functionmetal. The first Schottky contact metal layer is annealed at atemperature of at least 300° C. and at most 500° C. A second Schottkycontact metal layer is formed atop at least a portion of the firstSchottky contact metal layer.

A method of forming a Schottky diode and method of forming a fieldeffect transistor (FET) are carried out in accordance with this aspectof the invention.

Yet another aspect of the invention is a Schottky contact havingimproved metal adhesion. A first Schottky contact metal layer isdisposed atop at least a portion of a surface of a semiconductorstructure. The first Schottky contact metal layer includes a higher workfunction metal and is annealed at a temperature of at least 300° C. andat most 500° C. A second Schottky contact metal layer is disposed atopat least a portion of the first Schottky contact metal layer.

In accordance with this aspect of the invention, a Schottky diode and afield effect transistor (FET) are provided.

The foregoing aspects, features and advantages of the present inventionwill be further appreciated when considered with reference to thefollowing detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a cross-sectional view of a Schottky diode having a dualmetal Schottky contact according to an embodiment of the invention; FIG.1 b is a cut-away view showing the Schottky contact of the diode of FIG.1 a and the depletion region when no bias voltage is applied; and FIG. 1c is a cut-away view showing the Schottky contact of the diode of FIG. 1a and the depletion region when the device is reverse biased.

FIG. 2 shows a cross-sectional view of a Schottky diode having a dualmetal Schottky contact according to another embodiment of the invention;and FIGS. 2 a-2 e illustrate an example of self-aligned implant andself-aligned etch steps carried out in fabricating the diode of FIG. 2.

FIG. 3 a shows a cross-sectional view of a Schottky diode having a dualmetal Schottky contact according to a further embodiment of theinvention; FIG. 3 b is a cut-away view showing the Schottky contact ofthe diode of FIG. 3 a and the depletion region when no bias voltage isapplied; and FIG. 3 c is a cut-away view showing the Schottky contact ofthe diode of FIG. 3 a and the depletion region when the device isreverse biased.

FIG. 4 a shows a cross-sectional view of a field effect transistor (FET)having a dual metal Schottky contact according to a still furtherembodiment of the invention; FIG. 4 b is a cut-away view showing theSchottky contact region of the FET of FIG. 4 a and the depletion regionof the device with no bias voltage is applied; and FIG. 4 c is acut-away view showing the Schottky contact of the FET of FIG. 4 a andthe depletion region when the device is reverse biased.

FIG. 5 shows a cross-sectional view of a Schottky diode having a dualmetal Schottky contact according to yet another embodiment of theinvention.

DETAILED DESCRIPTION

The present invention provides a Schottky diode having two depositedSchottky contact metals to improve device performance when the device isreverse biased while maintaining substantially the same forward voltagedrop values when the device is forward biased. The first Schottkycontact metal has relatively small metal work function whereas thesecond Schottky contact metal has relatively high metal work function.When the device is forward biased, most of the Schottky contact has asmall barrier height because of the first Schottky contact metal'sreduced contact resistance which, as a result, improves current flow.When the device is reverse biased, the effect of the high work functionsecond Schottky contact metal dominates and results in a high reverseblocking voltage V_(R).

As used in the present disclosure, the term “III-V semiconductor” refersto a compound semiconductor material according to the stoichiometricformula Al_(a)In_(b)Ga_(c)N_(d)As_(e)P_(f) where (a+b+c) is about 1 and(d+e+f) is also about 1. The term “nitride semiconductor” or“nitride-based semiconductor” refers to a III-V semiconductor in which dis 0.5 or more, most typically about 0.8 or more. Preferably, thesemiconductor materials are pure nitride semiconductors, i.e., nitridesemiconductors in which d is about 1.0. The term “gallium nitride basedsemiconductor” as used herein refers to a nitride semiconductorincluding gallium, and most preferably including gallium as theprincipal metal present, i.e., having c≧0.5 and most preferably ≧0.8.The semiconductors may have p-type or n-type conductivity, which may beimparted by conventional dopants and may also result from the inherentconductivity type of the particular semiconductor material. For example,gallium nitride-based semiconductors having defects typically areinherently n-type even when undoped. Conventional electron donor dopantssuch as silicon (Si), germanium (Ge), sulfur (S), and oxygen (O), can beused to impart n-type conductivity to nitride semiconductors, whereasp-type nitride semiconductors may include conventional electron acceptordopants such as Magnesium (Mg) and Zinc (Zn).

FIG. 1 illustrates a cross-sectional view of a nitride-based Schottkydiode formed in accordance with the invention. The Schottky diode 100includes a substrate 102 upon which further layers are grown. Ideally,the substrate should have a lattice spacing, namely the spacing betweenadjacent atoms in its crystal lattice, that is equal to that of thegallium nitride or other nitride-based semiconductors that are to begrown atop the substrate to reduce the number of defects, such asdislocations in the crystal lattice, that are formed in thenitride-based semiconductor. Additionally, it is also highly desirablefor the substrate to have a thermal expansion coefficient at least equalthat of the nitride-based semiconductor so that when the substrate andnitride-based semiconductor are cooled after the growth of thenitride-based semiconductor layer, the substrate will contract more thanthe semiconductor layer, thereby compressing the semiconductor layer andavoiding the formation of cracks in the layer.

The substrate 102 may be an insulating or non-conducting substrate, suchas a crystalline sapphire wafer, a silicon carbide wafer or an undopedsilicon wafer, that is used to form a laterally conducting device. Tocompensate for the lattice mismatch and the thermal expansioncoefficient mismatch between the nitride-based semiconductor layer andthe substrates, a buffer layer (not shown) may be provided atop thesubstrate 102. The buffer layer may be comprised of one or more layersof nitride-based materials to provide a transition between the latticestructure of the substrate and the lattice structure of the galliumnitride or other nitride-based semiconductor layer.

A highly doped nitride-based semiconductor layer 104, such as galliumnitride or a gallium nitride-based semiconductor, is then formed atopthe buffer layer or, when the buffer layer is not present, directly atopthe substrate 102. The highly doped layer 104 is typically formed usingan epitaxial growth process. A reactive sputtering process may be usedwhere the metallic constituents of the semiconductor, such as gallium,aluminum and/or indium, are dislodged from a metallic target disposed inclose proximity to the substrate while both the target and the substrateare in a gaseous atmosphere that includes nitrogen and one or moredopants. Alternatively, metal organic chemical vapor deposition (MOCVD)is employed wherein the substrate is exposed to an atmosphere containingorganic compounds of the metals as well as to a reactivenitrogen-containing gas, such as ammonia, and dopant-containing gaswhile the substrate is maintained at an elevated temperature, typicallyaround 700-1100° C. The gaseous compounds decompose and form a dopedmetal nitride semiconductor in the form of a film of crystallinematerial on the surface of the substrate 102. The substrate and thegrown film are then cooled. As a further alternative, other epitaxialgrowth methods, such as molecular beam epitaxy (MBE) or atomic layerepitaxy may be used. The resulting highly doped layer 104 is preferablyn-type and has a doping concentration of at least 4E18 cm⁻³.

A lower doped nitride-based semiconductor layer 108, such as galliumnitride or a gallium nitride-based semiconductor, is formed atop atleast atop part of the highly doped layer 104, such as by modulationdoping. An example of a lower doped nitride-based semiconductor layerformed by modulation doping is described in U.S. application Ser. No.10/780,526, filed Feb. 17, 2004 to Pophristic et al., and titled “LowDoped Layer for Nitride-Based Semiconductor Device”, the disclosure ofwhich is incorporated herein by reference.

Typically, the lower doped layer 106 is formed atop the entire surfaceof the higher doped layer 104 and is then patterned and etched so thatportions of the lower doped layer 106 are removed and expose regions ofthe higher doped layer 104. Such patterning and etching steps may becarried out in a known manner.

To maximize the reverse blocking voltage V_(R) and minimize the forwardvoltage drop V_(F) of the Schottky diode, the dual metal Schottkycontact of the invention is used. A first Schottky metal layer 110 isformed atop the lower doped layer 106 in a known manner and forms themetal-to-semiconductor junction with the lower doped layer, known as aSchottky junction. The first Schottky contact metal layer 110 is formedof one or more metals having a relatively small metal work function,such as aluminum (Al), titanium (Ti), molybdenum (Mo), or gold (Au).Preferably, an Al contact metal layer is used to provide the lowestforward voltage drop.

A second Schottky contact metal 112 is then provided, preferably, atopof and surrounding the first Schottky contact metal 110. The secondSchottky contact metal layer is formed of one or more metals having arelatively high metal work function, such as nickel (Ni), palladium(Pd), a titanium-tungsten (TiW) alloy, tantulum (Ta), rhenium (Re),ruthenium (Ru) or platinum (Pt). Preferably, a Ni contact layer is usedto provide better device performance.

A shallow depletion region 120 is present in the portion of the lowerdoped layer 106 that adjoins the dual metal Schottky contact when nobias voltage is applied to the resulting device, as FIG. 1 b shows. Mostof the Schottky contact has a small barrier height because the firstSchottky contact metal 110 reduces the contact resistance and thusimproves the current flow.

When the device is reverse biased, a deeper depletion region is formed,shown in FIG. 1 c. The properties of the second Schottky contact metal112, namely, its high work function, dominates and results in a highreverse blocking voltage V_(R).

A further metal layer 108 is disposed atop the highly doped layer 104and forms an ohmic contact with the highly doped layer. The ohmic metallayer is typically a stack of one or more metals, such as analuminum/titanium/platinum/gold (Al/Ti/Pt/Au) stack or atitanium/aluminum/platinum/gold (Ti/Al/Pt/Au) stack, though other metalsor combinations of metals may be used. Examples of an Al/Ti/Pt/Au ohmiccontact stack and its formation are described in U.S. Pat. No.6,653,215, which is titled “Contact To n-GaN With Au Termination” andissued on Nov. 25, 2003, the disclosure of which is incorporated hereinby reference.

A thicker bond pad metal layer (not shown) may be formed atop theSchottky metal layers 110,112 and atop the ohmic metal layer 108. Thebond pad metal layer is typically a thick layer of aluminum (Al) or gold(Au). A passivation layer (not shown) comprised of an insulator may beformed at least between the ohmic metal and Schottky metal layers.

The Schottky metal layers 110,112 and the ohmic metal layer 106 may beformed using methods known in the art.

Another embodiment of a Schottky diode having the dual metal Schottkycontact of the invention is shown in FIG. 2. In this embodiment, a firstSchottky contact metal layer 210 has a low work function and is alsoused as an implant mask to permit a self-aligned ion implant to becarried out prior to forming a second Schottky contact metal layer 212.The second Schottky contact metal 212 has a high work function and alsoserves as a mesa etch mask during a self-aligned mesa etch of lightlydoped layer 206.

FIGS. 2 a-2 e illustrate an example of process steps carried out to formthe Schottky diode of FIG. 2. As FIG. 2 a shows, a highly dopednitride-based semiconductor layer 204 is formed atop a substrate 202.Next, a lower doped nitride-based semiconductor layer 206 is formed atopthe highly doped layer 204, and a first Schottky metal layer 210 is thenformed atop the lower doped layer 206 in the manner described aboveregarding FIG. 1.

Then, as FIG. 2 b shows, the first Schottky metal layer 210 is patternedand etched so that only a portion of the first metal layer remains atopthe lower doped layer 206. The remaining portion of the first Schottkymetal layer 210 is then used as a mask during an ion implant step toform implanted regions 214 in the lower doped layer 206, as FIG. 2 cshows. Typically, one or more species such as cadmium (Cd), magnesium(Mg), zinc (Zn), iron (Fe), nickel (Ni), silicon (Si), aluminum (Al),boron (B), nitrogen (N), or oxygen (O) are implanted at this time.

Then, a second Schottky metal layer 212 is formed atop of andsurrounding the first Schottky metal layer 210, as shown in FIG. 2 d.The second metal layer 212 may be formed by selective deposition or by adeposition step followed by patterning and etching of the metal layer.The second metal layer 212 is then used to mask an etch step thatremoves the exposed regions of the implanted layer 214 as well as bothan underlying regions of the lightly doped layer 206 and a portion of anunderlying region of the highly doped layer 204, as FIG. 2 e shows.Thereafter, an ohmic metal contact layer 208 is formed in the mannerdescribed above to obtain the device shown in FIG. 2.

In this embodiment, the Schottky metal layers 210 and 212 cover theentire top surface of the mesa 206. To prevent possible arcing orshorting between the Schottky contact and the ohmic contact, the ohmicmetal contact 208 is preferably substantially farther away from the mesathan is found in known devices to maintain a desired minimum distancebetween the Schottky and ohmic contacts.

FIG. 3 a shows a Schottky diode 300 that uses the dual metal contactconfiguration according to a further embodiment of the invention. Ahighly doped nitride-based semiconductor layer 304 is formed atop asubstrate 302, and a lower doped nitride-based semiconductor mesa 306 isformed atop the highly doped layer 304 in the manner described above. Afirst low work function Schottky contact metal layer 310 is thendeposited selectively as very narrow stripes with small separationsbetween the stripes. A second higher work function Schottky contactmetal layer 312 is then deposited atop of and in the gaps between thefirst Schottky contact metal stripes 310.

When no voltage is applied to the device, shallow depletion regions 320are formed in the layer 306 under the second Schottky contact metallayer 312, as FIG. 3 b shows. When the device is reverse biased, thediode's depletion regions 320 grow downward as well as expand underneaththe first Schottky contact metal stripes 310 and then overlap tocompletely cut-off the lower work function first metal layer 310, asshown in FIG. 3 c. Preferably, the spacing between the first Schottkycontact metal stripes 310 should be sufficiently small so that thecut-off occurs. The cut-off of the first Schottky metal layer furtherreduces the diode's leakage current and also enhances the reverseblocking voltage V_(R) while the forward voltage drop V_(F) remainsessentially unaffected. Based on capacitance vs. voltage measurements,the width of the first Schottky contact metal stripes 310 are preferablyin the range of microns (1-5 μm) with a similarly sized spacing.

Alternatively, the first Schottky contact metal layer may be selectivelydeposited as small spot or dot-shaped regions, each having a width inthe range of single microns and with sufficiently small separationsbetween them.

The dual metal Schottky contact of the invention is also applicable to afield effect transistor (FET) device shown in FIG. 4 a. Here, a firstnitride-based semiconductor layer 404, such as GaN, is formed atop asubstrate 402. Another nitride-based semiconductor layer 406, such asAlGaN, forms a heterojunction with a region of the first nitride-basedsemiconductor layer 404. A lower work function first Schottky metalcontact layer 410 covers part of the top surface of the layer 406, and ahigher work function second Schottky metal contact layer 412 is disposedatop of and surrounding the first Schottky metal contact layer 410. Anohmic metal contact layer 408 is disposed atop the layer 404 and isseparated from the first and second Schottky metal contact layers410,412 by the layer 406.

FIGS. 4 b and 4 c show a cut-away portion of the FET and illustrate theoperation of the dual Schottky metal contact. When the Schottky diode ofthe FET is unbiased, shallow depletion regions 420 are formed within thelayer 406 underneath the regions where the second Schottky contact metalcontacts the layer 406, as shown in FIG. 4 b. When the FET is operatedsuch that the Schottky diode is reverse biased, the depletion regions420 expand both downward into the layer 406 as well as underneath thefirst Schottky contact metal 410 in the manner described above, as FIG.4 c shows.

In another embodiment of the invention, a dual arrangement of Schottkycontact metals is used to improve metal adhesion, as shown in FIG. 5. Afirst Schottky contact metal layer 510 may be a high work function metalor a combination of such metals, such as Ni, Pd, TiW alloy, Ta, Ru, Reor Pt. Preferably, a Ni layer is used to provide better deviceperformance. The second Schottky contact metal layer 512 may be formedof Ni, Pd, TiW, Pt, Al, Ti, Mo, Au or a combination of these metals.After the deposition of the first Schottky metal layer 510, the layer isannealed at a temperature that is lower than that required for formationof an ohmic contact but which is high enough to improve adhesion of theSchottky contact. Typically, the anneal temperature is in the range of300 to 500° C. After the first Schottky contact metal is annealed, asecond Schottky contact metal is deposited.

The two-metal structure and annealing process of FIG. 5 could also beused for other devices that have a GaN or an AlGaN top layer, such asfor diodes or FETs.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A Schottky contact disposed atop a planar surface of a semiconductor,said Schottky contact comprising: a first Schottky contact metal layerthat includes a plurality of stripes each having its bottom surfacedisposed on said planar semiconductor surface and spaced apart from eachother; and a second Schottky contact metal layer having a bottom surfacedisposed on said planar semiconductor surface at least between adjacentones of and at least adjoining outermost ones of said plurality ofstripes of said first Schottky contact metal layer so that the bottomsurfaces of said plurality of stripes are co-planar with the bottomsurface of said second Schottky contact metal layer and the bottomsurfaces of said plurality of stripes, said first Schottky contact metallayer having a lower work function than that of said second Schottkycontact metal layer.
 2. A Schottky contact according to claim 1, whereinsaid semiconductor includes a nitride-based semiconductor.
 3. A Schottkycontact according to claim 1, wherein said semiconductor includes agallium nitride-based semiconductor.
 4. A Schottky contact according toclaim 1, wherein said semiconductor includes gallium nitride (GaN).
 5. ASchottky contact according to claim 1, wherein said planar semiconductorsurface includes a plurality of implanted regions that are locatedbeneath said second Schottky contact metal layer.
 6. A Schottky contactaccording to claim 1, wherein said first Schottky contact metal layer isselected from the group consisting of aluminum (Al), titanium (Ti),molybdenum (Mo), and gold (Au).
 7. A Schottky contact according to claim1, wherein said second Schottky contact metal layer is selected from thegroup consisting of nickel (Ni), palladium (Pd), a titanium-tungsten(TiW) alloy, tantalum (Ta), rhenium (Re), ruthenium (Ru) and platinum(Pt).
 8. A Schottky contact according to claim 1, wherein a given one ofsaid plurality of stripes of said first Schottky contact metal has awidth that is substantially equal to a distance between that stripe andan adjacent one of said plurality of stripes of said first Schottkycontact metal layer.
 9. A Schottky contact according to claim 1, whereindepletion regions form beneath said second Schottky contact metal layerin said semiconductor, and a spacing between adjacent ones of saidplurality of stripes of said first Schottky contact metal layer issufficiently small such that when said Schottky contact is reversebiased, said depletion regions overlap and cut off said plurality ofstripes of said first Schottky contact metal layer.
 10. A Schottkycontact according to claim 1, wherein each one of said plurality ofstripes of said first Schottky contact metal layer has a width within arange of about 1 to 5 μm, and a distance between adjacent ones of saidplurality of stripes is about 1 to 5 μm.
 11. A Schottky diode,comprising: a lower layer of nitride semiconductor disposed atop asubstrate; an upper layer of nitride semiconductor disposed atop atleast a portion of said lower layer of nitride semiconductor, said lowerlayer of nitride semiconductor being more highly doped than said upperlayer of nitride semiconductor; a Schottky contact disposed atop saidupper layer of nitride semiconductor according to claim 1, said planarsemiconductor surface being a top surface of said upper layer of nitridesemiconductor; and a further metal contact layer disposed atop saidlower layer of nitride semiconductor such that an ohmic contact isformed.
 12. A field effect transistor (FET), comprising: a lower layerof nitride semiconductor disposed atop a substrate; an upper layer ofnitride semiconductor disposed atop at least a portion of said lowerlayer of nitride semiconductor, said upper layer being a differentnitride semiconductor than said lower layer so that a heterojunction isformed between said layers; a Schottky contact disposed atop said upperlayer of nitride semiconductor according to claim 1, said planarsemiconductor surface being a top surface of said upper layer of nitridesemiconductor; and a further metal contact layer disposed atop saidlower layer of nitride semiconductor such that an ohmic contact isformed.
 13. A method of forming a Schottky contact atop a planar surfaceof a semiconductor, said method comprising: forming a first Schottkycontact metal layer that includes a plurality of stripes each having itsbottom surface disposed on said planar semiconductor surface and spacedapart from each other; and forming a second Schottky contact metal layerhaving a bottom surface disposed on said planar surface at least betweenadjacent ones of and at least adjoining outermost ones of said pluralityof stripes of said first Schottky contact metal layer so that the bottomsurfaces of said plurality of stripes are co-planar with the bottomsurface of said second Schottky contact metal layer, said first Schottkycontact metal layer having a lower work function than said secondSchottky contact metal layer.
 14. A method according to claim 13,wherein said semiconductor includes a nitride-based semiconductor.
 15. Amethod according to claim 13, wherein said semiconductor includes agallium nitride-based semiconductor.
 16. A method according to claim 13,wherein said semiconductor includes gallium nitride (GaN).
 17. A methodaccording to claim 13, further comprising: doping exposed regions ofsaid planar semiconductor surface using said first Schottky contactmetal as an implant mask prior to said step of forming said secondSchottky contact metal layer.
 18. A method according to claim 13,further comprising: etching a third portion of said semiconductorsurface using said second Schottky contact metal as an etch mask.
 19. Amethod according to claim 13, wherein said first Schottky contact metalis at least one metal selected from the group consisting of aluminum(Al), titanium (Ti), molybdenum (Mo), and gold (Au).
 20. A methodaccording to claim 13, wherein said second Schottky contact metal is atleast one metal selected from the group consisting of nickel (Ni),palladium (Pd), a titanium-tungsten (TiW) alloy, tantalum (Ta), rhenium(Re), ruthenium (Ru) and platinum (Pt).
 21. A method according to claim13, wherein a given one of said plurality of stripes of said firstSchottky contact metal has a width that is substantially equal to adistance between that stripe and an adjacent one of said plurality ofstripes of said first Schottky contact metal.
 22. A method according toclaim 13, wherein depletion regions form beneath said second Schottkycontact metal in said semiconductor, and a spacing between adjacent onesof said plurality of stripes of said first Schottky contact metal issufficiently small such that when said Schottky contact is reversebiased, said depletion regions overlap each other and cut off saidplurality of stripes of said first Schottky contact metal.
 23. A methodaccording to claim 13, wherein each one of said plurality of stripes ofsaid first Schottky contact metal layer has a width within a range ofabout 1 to 5 μm, and a distance between adjacent ones of said pluralityof stripes is about 1 to 5 μm.
 24. A method of forming a Schottky diode,said method comprising: forming a lower layer of nitride semiconductoratop a substrate; forming an upper layer of nitride semiconductor atopat least a portion of said lower layer of nitride semiconductor, saidlower layer of nitride semiconductor being more highly doped than saidupper layer of nitride semiconductor; forming a Schottky contact atopsaid upper layer of nitride semiconductor according to claim 13, saidplanar semiconductor surface being a top surface of said upper layer ofnitride semiconductor; and forming a further metal contact layer atopsaid lower layer of nitride semiconductor such that an ohmic contact isformed.
 25. A method of forming a field effect transistor (FET), saidmethod comprising: forming a lower layer of nitride semiconductor atop asubstrate; forming an upper layer of nitride semiconductor atop at leasta portion of said lower layer of nitride semiconductor, said upper layerbeing a different nitride semiconductor than said lower layer so that aheterojunction is formed between said layers; forming a Schottky contactatop said upper layer of nitride semiconductor according to claim 13,said planar semiconductor surface being a top surface of said upperlayer of nitride semiconductor; and forming a further metal contactlayer disposed atop said lower layer of nitride semiconductor such thatan ohmic contact is formed.
 26. A Schottky contact having improvedmetal-to-semiconductor adhesion, said Schottky contact comprising: afirst Schottky contact metal layer that includes a plurality of stripeseach having its bottom surface disposed on a portion of a planar surfaceof a semiconductor structure, said first Schottky contact metal layerincluding a higher work function metal and having improved adhesion tothe semiconductor surface by being annealed at a temperature of at least300° C. and at most 500° C.; and a second Schottky contact metal layerhaving a bottom surface disposed atop at least a portion of said firstSchottky contact metal layer and on said planar semiconductor surface atleast between adjacent ones of and at least adjoining outermost ones ofsaid plurality of stripes of said first Schottky contact metal layer sothat the bottom surfaces of said plurality of stripes are co-planar withthe bottom surface of said second Schottky contact metal layer and thebottom surfaces of said plurality of stripes.
 27. A Schottky contactaccording to claim 26, wherein said semiconductor includes anitride-based semiconductor.
 28. A Schottky contact according to claim26, wherein said semiconductor includes a gallium nitride-basedsemiconductor.
 29. A Schottky contact according to claim 26, whereinsaid semiconductor includes gallium nitride (GaN).
 30. A Schottkycontact according to claim 26, wherein said first Schottky contact metalis at least one metal selected from the group consisting of nickel (Ni),palladium (Pd), a titanium-tungsten (TiW) alloy, tantalum (Ta), rhenium(Re), ruthenium (Ru) and platinum (Pt).
 31. A Schottky contact accordingto claim 26, wherein said second Schottky contact metal is at least onemetal selected from the group consisting of nickel (Ni), palladium (Pd),a titanium-tungsten (TiW) alloy, aluminum (Al), titanium (Ti),molybdenum (Mo), and gold (Au).
 32. A Schottky diode, comprising: alower layer of nitride semiconductor disposed atop a substrate; an upperlayer of nitride semiconductor disposed atop at least a portion of saidlower layer of nitride semiconductor, said lower layer of nitridesemiconductor being more highly doped than said upper layer of nitridesemiconductor; a Schottky contact disposed atop said upper layer ofnitride semiconductor according to claim 26, said planar semiconductorsurface being a surface of said upper layer of nitride semiconductor;and a further metal contact layer disposed atop said lower layer ofnitride semiconductor such that an ohmic contact is formed.
 33. A fieldeffect transistor (FET), comprising: a lower layer of nitridesemiconductor disposed atop a substrate; an upper layer of nitridesemiconductor disposed atop at least a portion of said lower layer ofnitride semiconductor, said upper layer being a different nitridesemiconductor than said lower layer to form a heterojunction betweensaid layers; a Schottky contact disposed atop said upper layer ofnitride semiconductor according to claim 26, said planar semiconductorsurface being a surface of said upper layer of nitride semiconductor;and a further metal contact layer disposed atop said lower layer ofnitride semiconductor such that an ohmic contact is formed.